Waste materials can be classified by type, as gaseous, liquid, solid, and radioactive; and by source, as chemical, municipal, agricultural, urban, and nuclear. Many methods of treating wastes, either by converting them to useful by-products or by disposing of them, are in operation or under experimentation. Dumping in streams and rivers has long been illegal and ocean dumping has been prohibited since 1973. Methods of disposal or treatment include incineration (garbage, paper, plastics), precipitation (smoke, solid-in-liquid suspensions), adsorption (gases and vapors), chemical treatment (neutralization, ion-exchange, chlorination), reclamation of sulfite liquors (paper industry), reclaiming (paper, metals, rubber), compaction (urban wastes), bacterial digestion (sewage), comminution, and melting (glass, metals). Some cases may involve a combination of these methods.The following procedures should also be mentioned: (1) Flash pyrolysis of certain solid wastes yields synthetic fuel oil and other useful products. (2) Urban, animal, and agricultural wastes can be fermented by anaerobic bacteria to yield proteins, fuel oil, etc.; the possibilities of converting cannery and other food-processing wastes into protein-rich foods and feeds are being investigated. (3) Catalytic oxidation of exhaust emission gases; devices have been installed in cars since 1973 for air-pollution control. (4) Recovery of methane from manures. (5) Incorporation of special additives to certain plastics (polystyrene bottles) to render them biodegradable. (6) High-pressure hydrogenation of garbage to yield a low-sulfur combustible oil. (7) Deactivating radioactive wastes by adsorption or ion exchange, as well as by solidification and hydraulic fracturing; high-activity wastes are buried in steel-lined concrete tanks. (8) Catalytic oxidation of waste chlorinated hydrocarbons, with partial recovery of chlorine. (9) Incineration of semisolid and liquid wastes at sea in ships designed for that purpose.See Radioactive Waste; Chemical Waste; Sewage Sludge; Urban Waste.
